1. Field of the Invention
This invention relates very generally to telecommunications between earth transmitting-receiving stations via a satellite.
More exactly, the invention relates to a satellite radioelectric beam coverage reconfiguration method thereby dynamically managing beam coverages as a function of requests from earth stations, while maintaining beam coverages with minimum size to meet a power balance within the satellite. The radioelectric coverages of the satellite are reconfigured as a function of the variations in the traffic volume and the positions of stationary or moving earth stations using the satellite as radioelectric relay. This reconfiguration adapts the resources of the payload in the satellite to the characteristics of the traffic at each moment. The invention is especially suited for cases of sporadic traffic such as that generated by VSAT (Very Small Aperture Terminal) or mobile stations.
2. Description of the Prior Art
Initially the satellites were designed to route communications from one point to another, as via a cable, and the extended coverage of the satellite was utilized to set up long distance links. Thus the "Early Bird" satellite linked two stations on either side of the Atlantic Ocean. Owing to the limited performance of these satellites, earth stations equipped with large antennae and hence very costly, had to be used. The increase in size and power of the satellites has subsequently reduced the size of the earth stations and hence their cost and consequently multiplied the number of these stations. Another quality of the satellite resulting from its extended coverage and hence its capacity to diffuse or collect, has thus been utilized: instead of transmitting signals from one point to another, an emitter in a given station can emit to a large number of station receivers via the satellite or, conversely, a large number of station can emit to a central station.
The first designed satellites uses single-beam antennae which have certain drawbacks: either the satellite provides the coverage of a large earth surface but the satellite's antenna gain is limited by the aperture angle of the beam; or the satellite provides the coverage of a small earth surface and in this case the antenna gain is higher but the beam cannot cover stations that are geographically too distant. Thus with a single-beam satellite a choice must be made between firstly an extended coverage but with low antenna gain, and hence links with high error rates, and secondly, a reduced coverage with a high antenna gain but whereby stations that are too distant cannot be interconnected.
The multi-beam antenna techniques harmonize these two alternatives. The area coverage of the satellite is extended because it results from the juxtaposition of several beams, and each beam offers an antenna gain all the higher that the aperture of the beam is reduced. Typically the multi-beam antennae on board a satellite consist of radiating elements, and combined respectively with beam-forming networks. A beam-forming network supplying the radiating elements of an antenna further comprises phase-shifters and power dividers whereby the associated antenna can be configured in a special way. Both on receiving and on emitting the antenna is thus programmed for particular radiation patterns. A radiation pattern is notably expressed by a gain as a function of on an angle with respect to the antenna axis.
According to the prior art, two methods are chiefly used within the context of a multi-beam satellite.
According to a first method known as "scanning beam"-type, predetermined coverage areas are illuminated cyclically by a same antenna beam whose orientation is controlled by special control values programming a beam-forming network. The stations located in a given coverage area only emit or receive information when said area is illuminated by the beam. In the absence of any memorization unit on board the satellite, at least two beams are necessary at each time: one to establish an uplink from the emitters in the earth stations to the satellite, the other to establish a downlink from the satellite to the receivers in the earth stations.
According to a second known method, the proposal is to use on receiving and emitting a certain number of respective beams and allocate respective variable capacities to the beams by modifying the pass-band in each one of them. To do so a dynamic allocation of the payload channels is obtained between the beams as a function of the traffic request in these beams.
Thus, according to the prior technique, no beam coverage reconfiguration method is available to satisfy requests from earth stations as a function of their geographical positions and the traffic volumes.